This invention relates to particle detectors, and more particularly to a method and apparatus for measuring the size distribution of particles in an atmosphere.
As described in detail in our U.S. Pat. No. 4,847,503 issued July 11, 1989, and our application Ser. No. 325,921 filed Mar. 20, 1989, radon is a disintegrating by-product of radium which in high concentration levels can be dangerous to human health. Traditionally, radon progeny have been divided into two general groups or fractions. The first involved what was referred to as unattached progeny wherein airborne progeny were carried along by ultrafine aerosol particles having a size of ten nm or less. The second fraction was referred to as attached progeny wherein the progeny was attached to dust particles of a size larger than ten nm generally twenty nm or greater. Recent investigations have determined that the more traditional "bimodal" activity size distributions are no longer valid. Recently, the unattached fraction actually has been determined as an ultrafine particle mode in the 0.5 to 3.0 nm size range whose nature is dependent upon the gaseous environment surrounding the radon decay products. There have also been disclosed several indoor activity-weighted size distribution measurements indicating the presence of a nucleation mode as a significant airborne mode with a size range from three nm up to approximately 20 to 50 nm, depending on the chemistry and/or particle size distribution of the ambient air. The attached progeny or accumulation mode is generally now considered to be everything above the nucleation mode. The result is now to refer to a "trimodal" distribution in which the three modes are: unattached, nucleation, and attached or accumulation. Furthermore, it is sometimes convenient to refer to the unattached and nucleation modes as "ultrafine progeny".
The principal health threat from radon comes from the ultrafine progeny which have greater mobility over their attached counterparts and which are more readily deposited in the sensitive regions of the respiratory tract. The health risk posed by this exposure to ultrafine progeny thus requires that a greater understanding and knowledge of the entire range of unattached and nucleation progeny be determined. Therefore, it has become increasingly important to be able to map the size distribution of radon progeny in a particular environment. In order to be able to accurately asses bronchial doses from radon progeny with different activity-weighted size distributions, it has become necessary to provide detectors to record the characteristics of this trimodal distribution.
As discussed in detail in the aforementioned patents, various devices have been available for testing for radon and even for testing for radon progeny. However, except for our foregoing patents, the ratio of progeny present to radon gas as well as the ratio of the unattached progeny to the total progeny could only be inferred from average measurements. Our foregoing patents disclose two devices and methods for broadly measuring the unattached and attached fractions of radon progeny which greatly help to limit the wide variation in inferred levels from average measurements. The foregoing patents generally divide the unattached progeny at the ten nm level with the attached progeny being anything over that. As indicated above, it has subsequently been found that there is an interval of progeny due to nucleation and other factors that appears to be an intermediate mode between the unattached and accumulation modes now called nucleation mode with a size range as indicated above.
While the method and apparatus disclosed herein are directed specifically to detection of radon progeny attached to aerosol particles, broadly speaking our method and apparatus apply to aerosol particle detection by diffusion plating out in our novel apparatus in accordance with our novel methods. Our method and apparatus may be applied to particle size distribution problems, environmental analysis problems and related aerosol particle involved problems.